This invention relates to the preparation of metallic-alloy articles, such as titanium-alloy articles, without melting of the metallic alloy.
Metallic-alloy articles are fabricated by any of a number of techniques, as may be appropriate for the nature of the article. In one common approach, metal-containing ores are refined to produce a molten metal, which is thereafter cast. The ores of the metals are refined as necessary to remove or reduce the amounts of undesirable minor elements. The composition of the refined metal may also be modified by the addition of desirable alloying elements. These refining and alloying steps may be performed during the initial melting process or after solidification and remelting. After a metal of the desired composition is produced, it may be used in the as-cast form for some alloy compositions (i.e., cast alloys), or further worked to form the metal to the desired shape for other alloy compositions (i.e., wrought alloys). In either case, further processing such as heat treating, machining, surface coating, and the like may be utilized.
The production of metallic alloys may be complicated by the differences in the thermophysical properties of the metals being combined to produce the alloy. The interactions and reactions due to these thermophysical properties of the metals may cause undesired results. Titanium, a commercially important metal, in most cases must be melted in a vacuum because of its reactivity with the oxygen and nitrogen in the air. In the work leading to the present invention, the inventors have realized that the necessity to melt under a vacuum makes it difficult to utilize some desirable alloying elements due to their relative vapor pressures in a vacuum environment. The difference in the vapor pressures is one of the thermophysical properties that must be considered in alloying titanium. In other cases, the alloying elements may be thermophysically incompatible with the molten titanium because of other thermophysical characteristics such as melting points, densities, chemical reactivities, and tendency of strong beta stabilizers to segregate. Some of the incompatibilities may be overcome with the use of expensive master alloys, but this approach is not applicable in other cases.
There is therefore a need for an improved method to make alloys of titanium and other elements that present thermophysical melt incompatibilities. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for preparing an article made of an alloy of a metal such as titanium with a thermophysically melt-incompatible alloying element. The present approach circumvents problems which cannot be avoided in melting practice or are circumvented only with great difficulty and expense. The present approach permits a uniform alloy to be prepared without subjecting the constituents to the circumstance which leads to the incompatibility, specifically the melting process. Unintentional oxidation of the reactive metal and the alloying elements is also avoided. The present approach permits the preparation of articles with compositions that may not be otherwise readily prepared in commercial quantities. Master alloys are not used.
An article of a base metal alloyed with an alloying element is prepared by mixing a chemically reducible nonmetallic base-metal precursor compound of a base metal and a chemically reducible nonmetallic alloying-element precursor compound of an alloying element to form a compound mixture. The alloying element is preferably thermophysically melt incompatible with the base metal, but both thermophysically melt incompatible and thermophysically melt compatible alloying elements may be present. The method further includes chemically reducing the compound mixture to a metallic alloy, without melting the metallic alloy, and thereafter consolidating the metallic alloy to produce a consolidated metallic article, without melting the metallic alloy and without melting the consolidated metallic article.
The nonmetallic precursor compounds may be solid, liquid, or gaseous. The chemical reduction is preferably performed by solid-phase reduction, such as fused salt electrolysis of the precursor compounds in a finely divided solid form such as an oxide of the element; or by vapor-phase reduction, such as contacting vapor-phase halides of the base metal and the alloying element(s) with a liquid alkali metal or a liquid alkaline earth metal. The final article preferably has more titanium than any other element. The present approach is not limited to titanium-base alloys, however. Other alloys of current interest include aluminum-base alloys, iron-base alloys, nickel-base alloys, and magnesium-base alloys, but the approach is operable with any alloys for which the nonmetallic precursor compounds are available that can be reduced to the metallic state.
In another embodiment, a method for preparing an article made of titanium alloyed with an alloying element comprises the steps of providing a chemically reducible nonmetallic base-metal precursor compound of titanium base metal, and providing a chemically reducible nonmetallic alloying-element precursor compound of an alloying element that is thermophysically melt incompatible with the titanium base metal, and thereafter mixing the base-metal precursor compound and the alloying-element precursor compound to form a compound mixture. The method further includes chemically reducing the compound mixture to produce a metallic alloy, without melting the metallic alloy, and thereafter consolidating the metallic alloy to produce a consolidated metallic article, without melting the metallic alloy and without melting the consolidated metallic article. Other compatible features described herein may be used with this embodiment.
The thermophysical melt incompatibility of the alloying element with titanium or other base metal may be any of several types, and some examples follow. In the alloys, there may be one or more thermophysically melt incompatible elements, and one or more elements that are not thermophysically melt incompatible with the base metal.
One such thermophysical melt incompatibility is in the vapor pressure, as where the alloying element has an evaporation rate of greater than about 100 times that of titanium at a melt temperature, which is preferably a temperature just above the liquidus temperature of the alloy. Examples of such alloying elements include cadmium, zinc, bismuth, magnesium, and silver.
Another such thermophysical melt incompatibility occurs when the melting point of the alloying element is too high or too low to be compatible with that of titanium, as where the alloying element has a melting point different from (either greater than or less than) that of titanium of more than about 400xc2x0 C. (720xc2x0 F.). Examples of such alloying elements include tungsten, tantalum, molybdenum, magnesium, and tin. Some of these elements may be furnished in master alloys whose melting points are closer to that of titanium, but the master alloys are often expensive.
Another such thermophysical melt incompatibility occurs when the density of the alloying element is so different from that of titanium that the alloying element physically separates in the melt, as where the alloying element has a density difference with titanium of greater than about 0.5 gram per cubic centimeter. Examples of such alloying elements include tungsten, tantalum, molybdenum, niobium, and aluminum.
Another such thermophysical melt incompatibility is where the alloying element, or a chemical compound formed between the alloying element and titanium, chemically reacts with titanium in the liquid phase. Examples of such alloying elements include oxygen, nitrogen, manganese, nickel, and palladium.
Another such thermophysical melt incompatibility is where the alloying element exhibits a miscibility gap with titanium in the liquid phase. Examples of such alloying elements include the rare earths or rare-earth-like elements such as cerium, gadolinium, lanthanum, erbium, yttrium, and neodymium.
Another, more complex thermophysical melt incompatibility involves the strong beta stabilizing elements that exhibit large liquidus-to-solidus gaps when alloyed with titanium. Some of these elements, such as iron, cobalt, chromium, nickel, or manganese, typically exhibit eutectic (or near-eutectic) phase reactions with titanium, and also usually exhibit a solid state-eutectoid decomposition of the beta phase into alpha phase plus a compound. Other such elements, such as bismuth and copper, typically exhibit peritectic phase reactions with titanium yielding beta phase from the liquid, and likewise usually exhibit a solid state eutectoid decomposition of the beta phase into alpha phase plus a compound. Such elements present extreme difficulties in achieving alloy homogeneity during solidification from melting. This results not only because of normal solidification partitioning causing micro-segregation, but also because melt process perturbations are known to cause separation of the beta-stabilizing-element-rich liquid during solidification to cause macro-segregation regions typically called beta flecks.
Another thermophysical melt incompatibility involves the alkali and alkali-earth metals, such as lithium and calcium, that typically have very limited solubility in titanium alloys. Finely divided dispersions of these elements, for example beta calcium in alpha titanium, may not be readily achieved using a melt process.
These and other types of thermophysical melt incompatibilities lead to difficulty or impossibility in forming acceptable alloys of these elements in a conventional melting practice. The present approach, in which the metals are not melted at all during production or processing, circumvents the thermophysical melt incompatibility to produce good quality, homogeneous alloys.
Some additional processing steps may be included in the present process. In some cases, it is preferred that the compound mixture be compacted, after the step of mixing and before the step of chemical reduction. The result is a compacted mass which, when chemically reduced, produces a spongy metallic material. After the chemical reduction step, the metallic alloy is consolidated to produce a consolidated metallic article, without melting the metallic alloy and without melting the consolidated metallic article. This consolidation may be performed with any physical form of the metallic alloy produced by the chemical reduction, but the approach is particularly advantageously applied to consolidating of the pre-compacted sponge. Consolidation is preferably performed by hot pressing or hot isostatic pressing, extrusion, but without melting in each case. Solid state diffusion of the alloying elements may also be used to achieve the consolidation.
The consolidated metallic article may be used in the as-consolidated form. In appropriate circumstances, it may be formed to other shapes using known forming techniques such as rolling, forging, extrusion, and the like. It may also be post-processed by known techniques such as machining, heat treating, surface coating, and the like.
The present approach may be used to fabricate articles from the precursor compounds, entirely without melting. As a result, the characteristics of the alloying elements which lead to thermophysical melt incompatibility, such as excessive evaporation due to high vapor pressure, overly high or low melting point, overly high or low density, excessive chemical reactivity, strong segregation tendencies, and the presence of a miscibility gap, may still be present but cannot lead to inhomogeneities or defects in the final metallic alloy. The present approach thus produces the desired alloy composition of good quality, but without interference from these thermophysical melt incompatibilities that otherwise would prevent the formation of an acceptable alloy.
The present approach differs from prior approaches in that the metal is not melted on a gross scale. Melting and its associated processing such as casting are expensive and also produce some undesirably microstructures that either are unavoidable or can be altered only with additional expensive processing modifications. The present approach reduces cost and avoids structures and defects associated with melting and casting, to improve mechanical properties of the final metallic article. It also results in some cases in an improved ability to fabricate specialized shapes and forms more readily, and to inspect those articles more readily. Additional benefits are realized in relation to particular metallic alloy systems, for example the reduction of the alpha case defect for susceptible titanium alloys.
Several types of solid-state consolidation are known in the art. Examples include hot isostatic pressing, and pressing plus sintering, canning and extrusion, and forging. However, in all known instances these solid-state processing techniques start with metallic material which has been previously melted. The present approach starts with nonmetallic precursor compounds, reduces these precursor compounds to the initial metallic material, and consolidates the initial metallic material. There is no melting of the metallic form.
The preferred form of the present approach also has the advantage of being based in a powder-form precursor. Starting with a powder of the nonmetallic precursor compounds avoids a cast structure with its associated defects such as elemental segregation on a nonequilibrium microscopic and macroscopic level, a cast microstructure with a range of grain sizes and morphologies that must be homogenized in some manner for many applications, gas entrapment, and contamination. The present approach produces a uniform, fine-grained, homogeneous, pore-free, gas-pore-free, and low-contamination final product.
The fine-grain, colony-free structure of the initial metallic material provides an excellent starting point for subsequent consolidation and metalworking procedures such as forging, hot isostatic pressing, rolling and extrusion. Conventional cast starting material must be worked to modify and reduce the colony structure, and such working is not necessary with the present approach.
Another important benefit of the present approach is improved inspectability as compared with cast-and-wrought product. Large metallic articles used in fracture-critical applications are inspected multiple times during and at the conclusion of the fabrication processing. Cast-and-wrought product made of metals such as alpha-beta titanium alloys and used in critical applications such as gas turbine disks exhibit a high noise level in ultrasonic inspection due to the colony structure produced during the beta-to-alpha transition experienced when the casting or forging is cooled. The presence of the colony structure and its associated noise levels limits the ability to inspect for small defects to defects on the order of about {fraction (2/64)}-{fraction (3/64)} of an inch in size in a standard flat-bottom hole detection procedure.
The articles produced by the present approach are free of the colony structure. As a result, they exhibit a significantly reduced noise level during ultrasonic inspection. Defects in the {fraction (1/64)}, or less, of an inch range may therefore be detected. The reduction in size of defects that may be detected allows larger articles to be fabricated and inspected, thus permitting more economical fabrication procedures to be adopted, and/or the detection of smaller defects. For example, the limitations on the inspectability caused by the colony structure limit some articles made of alpha-beta titanium alloys to a maximum of about 10-inch diameter at intermediate stages of the processing. By reducing the noise associated with the inspection procedure, larger diameter intermediate-stage articles may be processed and inspected. Thus, for example, a 16-inch diameter intermediate-stage forging may be inspected and forged directly to the final part, rather than going through intermediate processing steps. Processing steps and costs are reduced, and there is greater confidence in the inspected quality of the final product.
The present approach is particularly advantageously applied to make titanium-base articles. The current production of titanium from its ores is an expensive, dirty, environmentally risky procedure which utilizes difficult-to-control, hazardous reactants and many processing steps. The present approach uses a single reduction step with relatively benign, liquid-phase fused salts or with liquid alkali metals. Additionally, alpha-beta titanium alloys made using conventional processing are potentially subject to defects such as alpha case, which are avoided by the present approach. The reduction in the cost of the final product achieved by the present approach also makes the lighter-weight titanium alloys more economically competitive with otherwise much cheaper materials such as steels in cost-driven applications.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.